KABUL (AFP) - The US-led coalition in Afghanistan confirmed Saturday that 13 civilians were killed in an operation against insurgents, with such casualties a major source of tension between Kabul and its allies. The US military at first said that 15 militants were killed in air strikes in the western province of Herat late Monday but local officials said six women and two children were among the dead. An Afghan army and coalition forces team visited the area after the local allegations, it said. "Coalition forces confirmed three militants and 13 non-combatants were killed during a coalition forces' operation near Gozara district, Herat province, February 17," the coalition said in a statement. It did not say who the 13 were. "We expressed our deepest condolences to the survivors of the non-combatants who were killed during this operation," said US Brigadier General Michael Ryan in the statement. The number of civilian casualties in military operations is one of the main sources of tension between President Hamid Karzai's government and the United States, Kabul's main military backer. The United Nations said this week that more than 2,000 civilians were killed in insurgency-linked violence last year, the highest civilian death toll since the 2001 ouster of the Taliban. About 40 per cent of them died in military action, it said. Meanwhile, three soldiers in the US-led coalition helping to fight a Taliban-led insurgency in southern Afghanistan have died after their patrol was hit by a bomb, the US military said late Friday. The three were killed on Friday in the southern province of Uruzgan, it said in a statement. It did not give their nationalities. "Three coalition service members died of wounds sustained from an improvised explosive device during a combat reconnaissance patrol in the Uruzgan province, Friday," it said. Many of the troops in Uruzgan - where Taliban have a strong presence - are Australian and Dutch nationals serving with the NATO-led International Security Assistance Forces. Soldiers in a separate US-dominated coalition work alongside ISAF and the growing international forces. The latest fatalities take to 39 the number of international troops to lose their lives in Afghanistan this year, most of them in attacks, according to the icasualties.org site that tracks the conflicts in Afghanistan and Iraq.